The correct answer to this open question is the following.
Dally's motive for sneaking into the movie for free is that Dally enjoys breaking the rules. He had the money to pay for the ticket but he likes to break the rules. This tells us about his personality that he is a rebel and likes to be against the establishment.
We are talking about the 1967 novel called "The Outsiders," written by S. G. Hinton. In 1983, famous director Francis Ford Coppola directed a movie with the same name. The story refers to the difficult life and behavior of working-class young people that belong to gangs.
Dally sneaks into the movies simply because he does like to follow the rules, even though the ticket was cheap. What this tells us about Dally is that he never likes to do things in a legal way.
